German patent document DE 10 2009 001 401 A1 discusses a braking system for a vehicle, which may be equipped with a generator. FIG. 1 shows a coordinate system to illustrate a traditional mode of operation of such a braking system having a generator. The abscissa of the coordinate system in FIG. 1 is time axis t.
Starting at a point in time t0′, a driver of a vehicle braked according to the traditional mode of operation actuates a brake actuating element, e.g., a brake pedal. A brake actuation distance s (in mm) by which the brake actuating element is adjusted therefore increases as of time t0′ starting from an initial brake actuation distance s0. Initial brake actuation distance s0 may be equal to zero, for example.
Between times t0′ and t1′ (time interval A′), the vehicle, which was previously driving at a vehicle velocity v (in m/s) equal to an initial velocity v0, is braked by purely regenerative braking. Therefore, a brake pressure buildup in at least one brake circuit of the vehicle between times t0′ and t1′ is prevented despite the fact that brake actuation distance s is not equal to initial brake actuation distance s0. This may be accomplished by temporarily storing the brake fluid, transferred out of a brake master cylinder, within at least one storage volume, for example. Furthermore, a generator is activated between times t0′ and t1′ in such a way that a generator braking torque M_gen (in Nm) not equal to zero (which may correspond to brake actuation distance s) is exerted on at least one wheel of the vehicle.
However, to protect the generator, it is usually necessary to reduce the applied generator braking torque M_gen to zero before the vehicle comes to a standstill. In the traditional mode of operation of the braking system shown in FIG. 1, a setpoint generator braking torque (not illustrated) which declines (almost) linearly over time between times t1′ and t2′ (time interval B′) is predefined for the generator. To replace the braking effect of the generator which is lost due to the reduction of generator braking torque M_gen, an (almost) linearly increasing setpoint brake pressure/target pressure p_setpoint (in bar) (starting from an initial brake pressure p0) should be built up in at least one brake circuit and/or at least one wheel brake cylinder of the braking system between times t1′ and t2′. For this purpose, the at least one pump is usually used with the aid of which the brake fluid is pumped out of the at least one storage volume of the braking system into the at least one wheel brake cylinder and/or the at least one brake circuit between times t1′ and t2′.
At the start of the brake pressure buildup in the at least one wheel brake cylinder and/or the at least one brake circuit, a dead volume of the hydraulic braking system must often be overcome first. Furthermore, a braking system often has a comparatively high elasticity in the presence of a relatively low brake pressure in the at least one brake circuit. Therefore, at the start of a brake pressure buildup in a braking system, a comparatively large brake fluid volume must be transferred/pumped in comparison with the increase in pressure in the at least one brake circuit and/or the at least one wheel brake cylinder.
To achieve an actual brake pressure p_actual (in bar) according to predefined setpoint brake pressure/target pressure p_setpoint, the at least one pump must thus pump a comparatively large brake fluid volume into the at least one brake circuit and/or into the at least one wheel brake cylinder between times t1′ and t1a′, during which the initial phase of the brake pressure buildup occurs. Thus, as shown on the basis of FIG. 1, a comparatively high setpoint rotational speed n_setpoint (in 1/min) is to be implemented by the at least one pump to induce actual brake pressure p_actual, which is approximated to setpoint brake pressure p_setpoint, in the at least one wheel brake cylinder and/or the at least one brake circuit. For example, setpoint rotational speed n_setpoint, which is to be implemented by the at least one pump, is also still comparatively high between times t1a′ and t1b′. Setpoint rotational speed n_setpoint may be greater than 1000 revolutions per minute, in particular greater than 1500 revolutions per minute, between times t1′ and t1a′ and/or between times t1a′ and t1b′. This means a relatively high pump efficiency for the at least one pump. Only between times t1b′ and t2′, when the linear range of the volume-pressure characteristic of the braking system has been reached, will setpoint rotational speed n_setpoint be able to decline to a stationary level.